S. Ravishankar

Multiple scattering of EM waves by dielectric spheres located in the near field of a source of radiation

An analysis of multiple scattering of electromagnetic (EM) waves by two loss-free dielectric spheres with radii greater than a wavelength and located in the bear field of a source of radiation is presented. The incident field is expressed in terms of spherical vector wave functions (SVWF). Translational and rotational addition theorems are employed to express the SVWF of the incident field in the coordinate system associated with the dielectric scatterer. Numerical computations are performed for obtaining the amplitude and phase patterns of fields multiply scattered by two loss-free dielectric spheres, whose centers are located on the boresight axis and in the nearfield of an open-ended circular cylindrical waveguide excited in its dominant mode. Numerically computed results show good agreement with measured results obtained from a systematic experimental study on forward scatter performed in the X -band.

Analysis of shaped beam dielectric lens antennas for mobile broadband applications

Dielectric lens antennas are inherently broadband and cost effective at producing highly shaped beams critical to mobile broadband applications (MBS). The paper presents an accurate analytical formulation based on spherical modal expansion (SME) of the near fields of small apertures radiating in the presence of a shaped dielectric lens. The analysis treats the lens as a scatterer. It utilizes techniques for radial translation and coordinate rotation of the small aperture spherical modal coefficients to align them to the phase center of the dielectric lens. The spherical modal coefficients of the scattered fields due to the lens are then obtained by application of boundary conditions. A sample computation is performed to demonstrate the approach and supported by an experimental result. The analysis is flexible enough to accommodate different types of small radiating apertures and different shapes for the dielectric lenses.

Probe uncompensated near-field to far-field transformation for scanning over an arbitrary surface

A new technique of transforming the probe uncompensated near-field data to the near or far field is presented, which is useful in nearfield antenna measurements, when the probe antenna scans over an arbitrary surface enclosing the test antenna. The technique presented is analytically simple and sufficiently accurate.

Radiation from aperture antennas radiating in the presence of a dielectric sphere

A more systematic and rigorous procedure for analyzing the radiation patterns of an open-ended wavegnide radiating in the presence of a dielectric sphere is presented. The analysis is based on the formula developed earlier [5] for radial translation of the spherical vector wave functions. An experimental program is described which confiras the validity of the analysis presented.

Application of wavelet for improvement of rate-reach performance in ADSL interference environment

This paper discusses the cancellation of near end crosstalk (NEXT) and far end crosstalk (FEXT) using a non-parametric wavelet denoising technique. Crosstalk noise is estimated using heuristic Steinpsilas unbiased risk estimate (SURE) and Bayesian risk minimization threshold function. The noise level estimated by a threshold function is used by thresholding rules to mitigate the effect of noise in the signal. A comparison of rates achieved with three thresholding techniques viz; soft thresholds, hard thresholds and a garrote threshold have been presented. Results obtained with AWGN, 24 ADSL NEXT and 24 ADSL FEXT disturbers show a significant improvement in rate reach performance upwards of 4 Mbps. The computational complexity in the wavelet denoising technique is independent of the number of crosstalk sources and hence can be suitable candidate for a practical deployment of mitigation of cross talk.

Computations of scattering by shaped dielectric lenses and a comparison with measurements

Scattered field computations of shaped dielectric lenses irradiated by an array, are presented using two techniques reported recently by the authors, (a) A Spheroidal modal expansion that can be expressed in terms of a linear combination of orthogonal spherical modes and (b) A hybrid of geometrical optics and spherical modal analysis to predict the EM fields over a near field spherical surface surrounding the lens. For a verification of the methods, comparisons have been made for lens-array combination with results obtained from an EM flow solver. Here we present a third method by sectioning the shaped lens and computing the fields on a measurement surface, by adding the fields contributed by the approximated spherical sections. Significantly, also presented is a set of computations using these techniques for spheroidal lenses irradiated by 1×2 and 2×2 arrays in C-band, and validated with measured results obtained in an anechoic chamber. There is a far better agreement for the computational methods with measured results, as compared to EM flow solvers. Further these methods need only a fraction of its time for computations and are as independent of lens shape.

Extraction of two wire and power line loop topology using customized genetic algorithms

Channel discovery by a Modem is critical to rate determination in 2-wire, smart grids and in-vehicle communications. A two-step hybrid method to extract open port 2-wire and power line topologies and facilitate channel model simulation using a biologically-inspired Genetic Algorithm (GA) iterative optimization is discussed. An initial approximate estimate of the loop topology is obtained from Correlation Time Domain Reflectometry (CTDR), a technique well-discussed in literature. Subsequently, a GA optimization working on the outputs of Frequency Domain Reflectometry (FDR) data of the loop progresses iteratively and converges to a more accurate topology. The GA defines a fitness function based on the measured FDR data and the computed FDR echo of successive loop section estimates obtained using the Roulette Wheel Selection mechanism. This was validated by actual FDR measurement tests on 2-wire loops and final topology results are compared with those obtained using an MMSE optimization scheme. For typical two-dimensional power line topologies, we present simulation results in a realistic channel in presence of impulse noise, using this two-step hybrid method. The aforementioned methods reuse most of the firmware modules in an embedded DSL modem, thus needing no additional equipment for measurements while ensuring rapid convergence.

A Cost Effective Smart Antenna for Mobile Broadband Systems

A cost effective implementation of a ‘Smart Antenna’ is limited due to the number and high cost of RF array elements, RF distribution as well as phasing elements. Shaped Dielectric Lenses perform the task of collimation, to obtain multiple beams in a fixed set of directions. Lenses are inherently broadband, lower in cost, easy to fabricate, have lower dimensional tolerances, and act as a radome for the electrically small primary radiators that are embedded inside or placed behind the lens.

Implementation of dual latency operation in VDSL2 with downstream power back off on DSP chip

VDSL2 employs a dual latency where voice and video traffic are directed over two separate bearer channels. In this paper a new scheme that partitions the tones over the two latency paths based on SNR with down stream power back off (DPBO) is presented. The bit loading is done from the obtained SNR profile using the standard rate adaptive water filling, accounting the fractional bits. Results include bit loading pattern, data rates for the two latency paths with DPBO and implementation of the same on the TMS DSP processor 6713.

Dual latency operation in VDSL2 with downstream power back off

The main advantage of down stream power back off (DPBO) in VDSL2 is coexistence with ADSL. The ADSL link bit error rate (BER) will not get affected by introducing VDSL2 in the same binder. In this paper we present a new scheme that partitions the tones over the two latency paths based on signal-to-noise (SNR) with DPBO included in the computation. The bit loading is done from the obtained SNR profile using the standard rate adaptive water filling algorithm. The data rate of the VDSL2 system is calculated by realigning the tone loading after accounting for the fractional bits. Simulation results include bit loading pattern and data rates for the two paths with DPBO. It is further implemented on the TMS DSP processor 6713 and results hence obtained are found to match with the simulation results.